On-chip impedance network with digital coarse and analog fine tuning

ABSTRACT

System and method for providing precision a self calibrating resistance circuit is described that provides for matching a reference resistor using dynamically configurable resistance networks. The resistor network is coupled to the connection, wherein the resistor network provides a configurable resistance across the connection. In addition, the resistor network comprises a digital resistor network and an analog resistor network. Also, the circuit includes control circuitry for configuring the configurable resistance based on a reference resistance of the reference resistor. The configurable resistance is configured by coarsely tuning the resistor network through the digital resistor network and fine tuning the resistor network through the analog resistor network.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of thecommonly owned, provisional patent application, U.S. Ser. No.61/650,949, entitled “ON-CHIP IMPEDANCE NETWORK WITH DIGITAL COARSE ANDANALOG FINE TUNING,” with filing date May 23, 2012, which is hereinincorporated by reference in its entirety

BACKGROUND

Conventional dynamic random access memory (DRAM) memory modulearchitectures typically include line termination resistors on themotherboard. Although the termination resistors on the motherboardreduce some reflections on the signal lines, they are unable to preventreflections resulting from the stub lines that connect to the DRAM onthe module. That is, termination resistors mounted on a motherboard donot prevent transmission line effects from the transmission line stubsbetween the resistors and the receiving device. For instance, a signalpropagating from the memory controller to the DRAM encounters animpedance discontinuity at the stub leading to the DRAM on the module.The signal that propagates along the stub to the DRAM is reflected backonto the signal line, thereby introducing unwanted noise into thesignal.

The introduced noise and the consequential signal degradations that arenot addressed by such off-die termination become more pronounced withhigher data rates and longer stub lengths. Larger, multi-drop systemscontaining multiple DRAM modules introduce even more reflections andconsequently add more reflective noise, thereby resulting in furthersignal degradation.

One solution is to provide on-die termination for transmission lines,instead of line termination on the motherboard. By placing thetermination resistance on the die itself rather than on the motherboard,the reflections resulting from discontinuities in the line aresignificantly reduced. This produces a cleaner signal and enables fasterdata rates.

However, problems may be presented when the transmission lines areterminated using on-die resistors. Process variations for manufacturingsemiconductor devices may result in fluctuating resistance values. Whenimplemented as an on-die termination resistance, this results inimproper termination valuation. Also, temperature and voltagefluctuations presented on the die will affect the resistance values ofthe semiconductors used for building resistors used for termination onthe die. This results in a variation of the on-die terminationresistance, which increases the noise and reflections on thetransmission line negatively affecting the signal transferred along thetransmission line.

What is needed is an on-die termination resistance circuit that accountsfor process variations in the manufacture of semiconductor devices, aswell as temperature and voltage fluctuations affecting devices used fortermination resistance on a die.

SUMMARY

Embodiments of the present invention generally relate to integratedcircuits, and more specifically to self calibration circuits and methodfor implementing the same for tuning a configurable resistor network toa reference resistor. In one embodiment, the self calibration circuit isused to match a reference resistance provided by a connection to aconfigurable resistor network that comprises a digital portion forcoarse tuning the configurable resistor network and an analog portionfor dynamic fine tuning of the configurable resistor network. In otherembodiments, systems and methods are described for providing precisionon-die termination of a connection that provides a signal from areference resistor.

A self calibrating resistance circuit is described that provides fortuning a configurable resistance to a reference resistance. In oneimplementation, the self calibrating resistance circuit is used toterminate a connection providing the reference resistance usingdynamically configurable on-die resistance networks. The resistornetwork is coupled to the connection, wherein the resistor networkprovides a configurable resistance across the connection. In addition,the resistor network comprises a digital resistor network and an analogresistor network. Also, the circuit includes control circuitry forconfiguring the configurable resistance based on a reference resistanceof the reference resistor. The configurable resistance is configured bycoarsely tuning the resistor network through the digital resistornetwork and dynamically fine tuning the resistor network through theanalog resistor network.

In another embodiment, a self calibrating resistance circuit isdescribed that provides for tuning a configurable resistance to areference resistance including a range selector for setting an initialresistance of the configurable resistance. A connection provides asignal from a reference resistor comprising a reference resistance. Thecircuit includes a resistor network coupled to the connection, whereinthe resistor network provides a configurable resistance across theconnection and comprises a digital resistor network and an analogresistor network. The digital resistor network provides for configuringa digital portion of the configurable resistance, and the analogresistor network provides for configuring an analog portion of theconfigurable resistance across the connection. The digital portion andthe analog portion are coupled in parallel. Also, the circuit includescontrol circuitry for configuring the configurable resistance based on areference resistance of the reference resistor. The configurableresistance is configured by coarsely tuning the resistor network throughthe digital resistor network and dynamically fine tuning the resistornetwork through the analog resistor network. The circuit also includes arange selector for configuring a range used for setting an initialresistance of the digital portion of the configurable resistance.

In another embodiment, a method for self calibrating a resistance isdescribed that provides for tuning a configurable resistance to areference resistance provided by a signal across a connection. Themethod includes turning a resistor network that provides a configurableresistance across the connection, and comprises a digital resistornetwork and an analog resistor network coupled in parallel. The methodincludes coarsely tuning the digital resistor network to configure thedigital portion of the resistor network. In addition, fine tuning of theanalog resistor network is performed to configure the analog portion ofthe configurable resistance. The configurable resistance is related toor based on the reference resistance provided by the connection.

These and other objects and advantages of the various embodiments of thepresent disclosure will be recognized by those of ordinary skill in theart after reading the following detailed description of the embodimentsthat are illustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification and in which like numerals depict like elements,illustrate embodiments of the present disclosure and, together with thedescription, serve to explain the principles of the disclosure.

FIG. 1 is a diagram illustrating on-die termination of a transmissionline, in accordance with one embodiment of the disclosure.

FIG. 2 is a schematic illustrating a self calibration resistance circuitincluding a TERM_P resistance network for terminating a line to a powersupply, such as V_(DD), and an TERM_N resistance network for terminatinga line to ground or Vss, in accordance with one embodiment of thedisclosure.

FIG. 3A is a circuit diagram of a comparator in a TERM_P resistancecircuit used in a self calibrating resistance circuit for coarsely anddigitally converging a digital portion of a resistor network, inaccordance with one embodiment of the disclosure.

FIG. 3B is a circuit diagram of a comparator in a TERM_P resistancecircuit used in a self calibrating resistance circuit for finely tuningan analog portion of the resistor network of FIG. 3A, wherein the analogportion is coupled in parallel with the digital portion, in accordancewith one embodiment of the present disclosure.

FIG. 3C is a circuit diagram of a comparator in an TERM_N resistancecircuit used in a self calibrating resistance circuit for finely tuningan analog portion of the TERM_N resistance network, referenced to groundor Vss, in accordance with one embodiment of the present disclosure.

FIG. 4A is a circuit diagram of a comparator in a TERM_N resistancecircuit, referenced to a power supply, and used in a self calibratingresistance circuit for coarsely and digitally converging a digitalportion of a resistor network, in accordance with one embodiment of thedisclosure

FIG. 4B is a circuit diagram of a comparator in a TERM_N resistancecircuit used in a self calibrating resistance circuit for finely tuningan analog portion of the resistor network of FIG. 4A, wherein the analogportion is coupled in parallel with the digital portion, in accordancewith one embodiment of the present disclosure.

FIG. 5A is a flow diagram illustrating a method for self calibrating aresistor network comprising a digital portion and an analog portion to areference resistor, in accordance with one embodiment of the disclosure.

FIG. 5B is a flow diagram illustrating a method for digitally converginga digital portion of the resistor network of FIG. 5A, in accordance withone embodiment of the present disclosure.

FIG. 6 is a circuit diagram of a comparator circuit used in a selfcalibrating resistance circuit, in accordance with one embodiment of thedisclosure.

FIG. 7 is a circuit diagram of a reference array for configuring adigital portion of a configurable resistor network that is used in aself calibrating resistance circuit, in accordance with one embodimentof the disclosure.

FIG. 8 is a table illustrating a range select feature of the selfcalibrating resistance circuit, in accordance with one embodiment of thedisclosure.

FIG. 9A is a circuit diagram of a TERM_P resistance network forterminating a line to a positive power supply, such as V_(DD), inaccordance with one embodiment of the disclosure.

FIG. 9B is a circuit diagram of an TERM_N resistance network forterminating a line to ground or Vss, in accordance with one embodimentof the disclosure.

FIG. 10 is a circuit diagram of a resistor control block used forconfiguring the digital and analog portions of the configurable resistorin a self calibrating resistance circuit, in accordance with oneembodiment of the disclosure.

FIG. 11 is a signal diagram illustrating waveforms of simulation resultswhen implementing a self calibrating resistance network, in accordancewith one embodiment of the present disclosure.

FIG. 12 is a circuit diagram illustrating a resistance step down featureof the self calibrating resistance circuit, in accordance with oneembodiment of the disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. While described in conjunction with theseembodiments, it will be understood that they are not intended to limitthe disclosure to these embodiments. On the contrary, the disclosure isintended to cover alternatives, modifications and equivalents, which maybe included within the spirit and scope of the disclosure as defined bythe appended claims. Furthermore, in the following detailed descriptionof the present disclosure, numerous specific details are set forth inorder to provide a thorough understanding of the present disclosure.However, it will be understood that the present disclosure may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail so as not to unnecessarily obscure aspects of the presentdisclosure.

Accordingly, embodiments of the present disclosure provide a circuit andmethod for self calibrating to a reference resistor. Embodiments of thepresent invention are disclosed within the context of integratedcircuits and self calibrating resistance circuitry that is implementablefor various functions, such as the tuning to a reference resistanceprovided by a connection or transmission lines to the integratedcircuit. For instance, the integrated circuit may include a memory arraystructure (e.g., DRAM memory array), wherein the self calibratingresistance circuit and method for implementing the same is used toterminate a transmission line to the memory array.

FIG. 1 is a diagram 100 illustrating on-die termination of atransmission line, in accordance with one embodiment of the invention.By placing the termination resistance on the die itself, rather than onthe motherboard, the reflections resulting from discontinuities in thetransmission line are significantly reduced. This produces a cleanersignal and enables faster data rates.

As shown in FIG. 1, a signal propagates from a driver 120 (e.g., memorycontroller) to a device 140, such as a memory array structure (e.g.,DRAM memory array). Instead of a having an off-die terminationresistance that introduces a second stub and its related impedancediscontinuities, a single stub 110 is introduced between the memorycontroller 120 and the device 140. Stub 110 represents the transmissionline between the driver 120 and the device 140. As such, a terminationresistance 150 is able to match the resistance presented over thetransmission line as represented by the single stub 110.

In embodiments of the present invention, on-die termination isimplemented with several combinations of resistors to form resistance150 on the die. For instance, on-die termination is implemented with aconfigurable resistor array including a digital portion providing coarsetuning of the configurable resistor and an analog portion providing finetuning of the configurable resistor. The final resistance of theconfigurable resistor is determined by an external reference resistor,such as that represented by sub 110. As such, the configurable resistoris configured to create a proper equivalent impedance value to theoutside of the die.

FIG. 2 is a schematic illustrating a self calibration resistance circuit200 for self calibrating a configurable resistor to a reference resistorprovided over a connection, in accordance with one embodiment of theinvention. In one implementation, the configurable resistor is used foron-die line termination. That is, the resistance circuit 200 is includedwithin an integrated circuit that is coupled to the connection. In thatcase, the configurable resistor comprises an internal terminationresistance, and the reference resistor comprises an external referenceresistor, such as that provided over the connection.

Circuit 200 includes a resistor network that is coupled to a connection(not shown), wherein the connection provides a signal from referenceresistor. The resistor network provides a configurable resistance acrossthe connection, wherein the resistor network comprises a digitalresistor network 250 and an analog resistor network, wherein the analogresistor network is configurable through the TERM_P resistance network270 and/or the TERM_N resistance network 275. The digital resistornetwork 250 is used to coarsely tune the resistor network through adigital portion, whereas the analog resistor network is used to finelytune the resistor network through an analog portion.

As shown in FIG. 2, the reference array 250 provides a network ofdigital resistive switches arranged in a parallel configuration that isused to configure the digital portion of the configurable resistance.The digital resistor network is configured to coarsely tune theconfigurable resistance.

In one implementation, the self calibration resistance circuit 200Bincludes an analog resistor network that is configured to fine tune theconfigurable resistance. More particularly, for analog fine tuning, theself calibration resistance circuit 200 includes a TERM_P resistancenetwork 270 that is used for configuring a resistance referenced to apower supply, such as V_(DD), (e.g., the resistance may be used forterminating a line), and a separate and independent TERM_N resistancenetwork 275 for configuring a resistance referenced to ground or Vss(e.g., the resistance may be used for terminating a line), in accordancewith embodiments of the invention.

As such, the TERM-P resistance provides for an analog fine tuning of theconfigurable resistance for resistances referenced to V_(DD). That is,the TERM_P resistance network 270 provides for an analog portion that isdynamically tuned and configured in parallel with the digital portion ofthe configurable resistance. Also, the TERM_N resistance network 275provides for analog fine tuning of the configurable resistance forresistances referenced to ground or Vss. That is, the TERM_N resistancenetwork 275 provides for an analog portion that is dynamically tuned andcoupled in parallel with the digital portion of the configurableresistance.

In one embodiment, the TERM_N resistance network 275 is configured usingthe reference signals generated from the TERM-P resistance network 270.That is, the TERM_P resistance network 270, or its copy, acts as thereference resistance that is used to configure the TERM_N resistancenetwork 275. In one embodiment, the TERM_N resistance network 275 ismirrored to the p-term resistance network 230. That is, a current mirrorbias mirrors the bias from the analog FTP signal generated from theTERM_P resistance circuit 270 to form a mirrored FTN signal. The TERM_Nresistance network 275 provides for an analog fine tuning of theconfigurable resistance for resistances referenced to or terminating toground or Vss.

The self calibrating resistance circuit 200 also includes controlcircuitry for dynamically configuring a configurable resistor thatcomprises the digital portion 250 and the analog portion 270 and/or 275.For instance, control circuitry includes the comparator 255 and theresistance control block 260 acting together. The configuration of theconfigurable resistor is based on a reference resistance of thereference resistor and is accomplished by fine tuning the resistornetwork through the digital resistor network and fine tuning theresistor network through the analog resistor network.

More particularly, the self calibrating resistance circuit 200 iscapable of establishing a configurable resistance or impedance that isproportional to a reference resistor. In one implementation, theresistance circuit 200 is capable of establishing a configurableimpedance that is proportional to an external precision resistor. In oneembodiment, the proportion is such that the configurable resistance orimpedance is substantially identical to the resistance of the referenceresistor. In one embodiment, the reference resistance is 240 ohms, whichis characteristic of a termination resistance of a DRAM memorystructure.

The configurable resistance is configured through combinations ofswitchable resistors whose final value is tuned to the referenceresistor. The configurable resistor comprises a digital portion and ananalog portion configured in parallel, wherein the digital portioncoarsely tunes the configurable resistance, and the analog portionfinely tunes the configurable resistance. That is, the self-calibratingcircuit 200 is capable of establishing a configurable resistance that isproportional to a reference resistor, such as an external precisionresistor. For instance, in a DRAM memory array structure, on-dietermination is implemented to configure the combinations of switchableresistors on the DRAM die, whose final value is determined by anexternal reference resistor. In addition, the reference resistor is alsoused for output driver calibration, in another embodiment.

The self calibrating resistance circuit 200 acts in a digital mode andan analog mode. In the digital mode, the circuit 200 forces the TERM_Pand TERM_N resistance circuits into an open circuit state. That is, theanalog portion of the configurable resistance is not configured. Forinstance, when EN is inactive, the circuit forces the TERM Rs(TERM_P/TERM_N) into an open circuit state. TERM_P resistance circuit270 has one terminal connected to positive supply (V_(DD)) while TERM_Nresistance circuit 275 has one terminal connected to Vss. With EN activeand RSETB removed the clock starts, and after a max of 10 system clocks,the TERM Rs values are set, in one embodiment. As such, in the digitalmode, the digital portion of the configurable resistance is establishedthrough the reference array 250, wherein the reference array 250comprises a plurality of resistive branches coupled in parallel.

This is accomplished by comparing the divider voltage ratio of REF_ARRAYvalue/R_REF voltage to an internal divider voltage using the comparator255. A shift register shifts left or right depending on the output ofthe comparator 255, and is used to enable or disable a resistive branchof the reference array 250. For instance, shifting in a one enablesanother parallel resistor bank (all of which are in parallel with theinitial range select value) while shifting in a zero disables a resistorbank.

In one embodiment, the self calibrating resistance circuit 200 providesfor on-die termination to an integrated circuit (e.g., a DRAM memoryarray structure). As such, the on-die termination (ODT) calibration is atechnique that involves calibrating the termination impedance in orderto optimize the reduction of signal reflections. In various embodimentsof the present disclosure, ODT calibration allows establishing anoptimal termination value that compensates for variations in process andoperating conditions. More particularly, the integrated circuitcomprises the resistor network and the control circuitry, and whereinthe integrated circuit is coupled to the connection. The configurableresistance comprises an internal termination resistance. For instance,the termination resistance is matched to an external referenceresistance presented over a transmission line or connection.

FIG. 3A is a circuit diagram of a comparator circuit 300A used forconfiguring a digital portion of a configurable resistor that isreferenced to a positive power supply (e.g., V_(DD)), in accordance withone embodiment of the invention. That is, comparator circuit 300A isused in a self calibrating resistance circuit for coarsely and digitallyconverging a digital portion of a resistor network. In oneimplementation comparator circuit 300A is used within the comparator 255of FIG. 2. In another implementation, comparator circuit 300A is usedwithin comparator circuit 600 of FIG. 6.

Specifically, the output of the comparator circuit 300A is used toconfigure the digital portion of the configurable resistor. This isaccomplished by comparing divider voltages between a fixed dividervoltage and a second divider voltage. The comparator circuit 300A isimplementable within the comparator 255 of FIG. 2, in one embodiment. Inparticular, a fixed voltage divider 310 comprises a fixed dividervoltage measurable at node 315. The fixed voltage divider comprises afirst resistive component and a second resistive component. In oneimplementation, the fixed voltage divider includes resistors 305 with anode 315 that is used as an input 353 to the comparator 330. In oneembodiment, the resistors in the fixed voltage divider 310 comprises aone-to-one ratio, wherein the resistors 305 are substantially identical.In another embodiment, the resistors in the fixed voltage divider 310comprises a fixed ratio that is not one-to one, wherein the resistorsare not identical.

Another input 335 to the comparator 330 is coupled to a second voltagedivider 320 that comprises a second divider voltage measured at node327. The second voltage divider 320 includes a reference resistor 325,such as an external precision reference resistor. In addition, thesecond voltage divider 320 includes a configurable resistor thatincludes a digital portion 321 that is configured through comparatorcircuit 300A. For purposes of coarse tuning, the digital portion isused, wherein the digital portion comprises a plurality of selectableresistive branches arranged in a parallel configuration. Adding aresistive branch reduces the resistance of the digital portion 321,whereas removing a resistive branch increases the resistance of thedigital portion 321.

The comparator 330 is configured to compare the second divider voltageand the fixed divider voltage to generate an output signal fromcomparator 330. More particularly, the output signal is delivered to theresistor control block 322 in the digital mode to configure the digitalportion of the configurable resistance. In one embodiment, resistorcontrol block 322 performs the same functions as resistor control block260 of FIG. 2. For example, in the comparator circuit 600 of FIG. 6, theCOMP signal is provided as an output of the comparator 610, acting inthe digital mode, that is delivered to the resistor control block 322.The COMP signal is used to configure the digital portion of theconfigurable resistance.

In one embodiment, the resistor control block 322 includes digitalconvergence circuitry for adding or removing one of the plurality ofparallel resistive branches used for configuring the digital portion ofthe configurable resistor as determined by the output signal. In anotherembodiment, the resistor control block 322 includes a register foradding or removing one of the plurality of parallel resistive branchesused for configuring the digital portion of the configurable resistor asdetermined by the output signal. In still another embodiment, theregister comprises a shift register for adding or removing one of theplurality of parallel resistive branches used for configuring thedigital portion of the configurable resistor as determined by the outputsignal.

In particular, the digital convergence circuitry adds a resistive branchwhen and as long as the output signal indicates said second dividervoltage measured at node 327 is lower than the fixed divider voltagemeasured at node 315. On the other hand, the digital convergencecircuitry removes a digital resistive branch when the output signalindicates the second divider voltage measured at node 327 is higher thanthe reference divider voltage measured at node 315. By starting at aresistance value that is higher than the reference resistor, the selfcalibrating resistance circuit is able to reduce the digital portion ofthe configurable resistance through the addition of parallel digitalresistive branches.

At a point when the output signal first indicates that the seconddivider voltage measured at node 327 is higher than the referencedivider voltage measured at node 315, the resistor control block 322removes a digital leg from the digital portion and then locks thedigital portion of the configurable resistor. At that time, the voltagemeasured at node 327 is again lower than the voltage at node 315,thereby indicating that the digital portion of the configurableresistance is again higher than the reference resistance. That is, thedigital convergence circuitry digitally converges after a last resistivebranch is added, which results in the output signal from the comparator330 indicating that the second divider voltage at node 327 is higherthan the fixed divider voltage at node 315, and when the last resistivebranch is thereafter removed so that the configurable resistance isagain greater than the reference resistance, which results in the outputsignal from the comparator 330 indicating that the voltage at node 327is again lower than the voltage at node 315.

In one embodiment, the digital convergence circuitry digitally convergesbased on a clock signal. That is, legs are added or removed according toa clock signal. In one embodiment, the clock signal comprises anexternal clock signal. In another embodiment, the self calibrationresistance circuit internally generates the clock signal. For instance,a ring oscillator may be used for generating the clock signal, in oneembodiment.

For example, FIG. 7 is a circuit diagram of a reference array 700 forconfiguring a digital portion of a configurable resistor network, inaccordance with one embodiment of the disclosure. As shown, referencearray 700 has N selectable resistor banks 750 that are controlled by theresistor control block 260, 322, and/or 422. Resistor banks are addedsingly in parallel depending on the output of the resistor controlblock. After the resistor banks 750, an END control cell 760 guaranteesthat the circuit switches to FUNE TUNE mode in all cases. The last blockon the right is the SW_RES_FT and is an analog adjust for fine tuningthe resistance and allows for compensation if voltage or temperaturechanges.

Thereafter, analog fine tuning is performed in order to dynamicallyadjust the configurable resistance. In one implementation, the digitalconvergence circuitry, registers, and/or shift registers converges tothe digital portion before the analog fine tuning is performed. Inreference to FIG. 2, the self calibrating circuit 200 fine tunes theconfigurable resistance through dynamically tuning the analog portion.In one embodiment, for purposes of fine tuning, the analog portion isused in conjunction with the digital portion, and comprises a transistor(e.g., metal oxide semiconductor) that has a variable resistancedepending on a gate to source voltage difference.

FIG. 3B is a circuit diagram of a comparator circuit 300B, or TERM_Presistance circuit, for configuring an analog portion of theconfigurable resistor, in accordance with one embodiment of theinvention. In particular, the comparator 350 in the TERM_P resistancecircuit is used in a self calibrating resistance circuit for finelytuning an analog portion of a resistor network, wherein the analogportion is coupled in parallel with the digital portion as configured inFIG. 3A, in accordance with one embodiment of the present disclosure.Specifically, the output of the comparator circuit 300B is used toconfigure the analog portion of the configurable resistor through theTERM_P resistance circuit (e.g., 270) in order to configure a resistancethat is referenced to a positive power supply (e.g., V_(DD)). This isaccomplished by comparing divider voltages for a TERM_P resistancecircuit. In one embodiment, comparator 350 is the same comparator 330 ofFIG. 3A, wherein in other embodiments, comparator 350 is independent ofcomparator 330. Comparator circuit 300B is implementable withincomparator 255 of FIG. 2, in one embodiment.

In particular, fixed voltage divider 310 includes resistors 305 with anode 315 that provides a fixed divider voltage. In one embodiment,resistors 305 are substantially identical, wherein in other embodiments,resistors in the fixed voltage divider 310 have different resistances.The fixed divider voltage provided at node 315 is also provided as aninput 353 to comparator 350.

Another input 355 to comparator 350 is coupled to a second voltagedivider 340 that comprises a second divider voltage measured at node327. The second voltage divider 340 includes the reference resistor 325,and a configurable resistor. More particularly, the second voltagedivider includes a node 347 that is used as an input 355 to thecomparator 350. The configurable resistor includes the digital portion321 that is configured through comparator circuit 300A, and an analogportion 343 that comprises a variable resistive branch coupled inparallel with the digital portion 321.

Comparator circuit 300B also includes an operational amplifier 350.Inputs to the operational amplifier includes a fixed divider voltagefrom the fixed voltage divider, and a second divider voltage from thesecond voltage divider. The operational amplifier 350 generates a biasvoltage at an output. A capacitor 358 coupled to the output acts as afilter.

Specifically, as shown in FIG. 3B, the analog portion of theconfigurable resistance is dynamically tuned through the TERM_Presistance network 270. More particularly, the feedback loop 359 isenabled to fine tune the analog portion 343 of the configurableresistance, such that that the divider voltage provided by voltagedivider 340 matches that provided by voltage divider 310. This isaccomplished by feeding the bias voltage back to the second voltagedivider for adjusting the analog portion of the configurable resistance.In particular, the bias voltage of the operational amplifier 350 is fedback to the variable resistive branch of the analog portion 343. Assuch, the analog fine tune is responsive to further fluctuations intemperature and voltage that affect the self calibrating resistancecircuits 200A and 200B. For instance, in FIG. 6, the FTP signal providesan output of the comparator that is used to configure the analog portion343 of the configurable resistance.

In one embodiment, the variable resistive branch of the analog portioncomprises a transistor, wherein the transistor comprises a variableresistance as determined by a variable gate voltage. The gate voltage iscontrolled by a bias voltage provided by the comparator that iscomparing divider voltages between the fixed voltage divider 310 and thesecond voltage divider 340. That is, the output of the comparator oroperational amplifier is coupled to the transistor. In one embodiment,the transistor operates in the linear range when configuring the analogportion of the configurable resistance.

FIG. 3C is a circuit diagram of a comparator circuit 300C, or TERM_Nresistance circuit, for configuring an analog portion of theconfigurable resistor, in accordance with one embodiment of theinvention. In particular, the comparator 390 in the TERM_N resistancecircuit is used in a self calibrating resistance circuit for finelytuning an analog portion of a resistor network, wherein the analogportion is coupled in parallel with the digital portion as configured inFIG. 3A, in accordance with one embodiment of the present disclosure.Specifically, the output of the comparator circuit 300B is used toconfigure the analog portion of the configurable resistor through theTERM_P resistance circuit (e.g., 270) in order to configure a resistancethat is referenced to a negative power supply or ground (e.g., Vss).That is, the TERM_N resistance network 275 provides for an analog finetuning of the configurable resistance for resistances referenced to orterminating to ground or Vss. This is accomplished by comparing dividervoltages for a TERM_N resistance circuit. Comparator circuit 300C isimplementable within comparator 255 of FIG. 2, in one embodiment.

In one embodiment, comparator circuit 300C is a complementary circuitthat uses control signals generated from the comparator circuit 300B inorder to configure a resistance that is referenced to ground or Vss. Inparticular, the comparator circuit 300C comprises a second resistornetwork 365 that provides a second configurable resistance that isreferenced to ground or Vss. The second resistance network 365 isconfigured similarly to include a digital portion 371 and an analogportion 383. The digital portion 371 is configured similarly to thedigital portions 321 of FIGS. 3A and 3B. Specifically, the outputs ofthe resistor control block 322 is used to configure the digital portion371 of the second resistor network 365.

Once the digital portion 371 is configured and set, analog fine tuningis performed. That is, second control circuitry is used for configuringthe second configurable resistance that is based on a second referenceresistance, wherein the second reference resistance is obtained fromcontrol signals generated by the TERM_P resistance circuit 300B. Thatis, the second reference resistance is configured similarly as theresistor network used to configure the configurable resistance (e.g.,comprising digital portion 321 and analog portion 343) of FIG. 3B. Thisis accomplished by fine turning the second analog resistor network 363.

Specifically, the TERM_N resistance network 275 is configured using thereference signals generated from the TERM-P resistance network 270 or300B. That is, the configurable resistance generated through the TERM_Presistance network 270 or 300B, or its copy, acts as the referenceresistance configuration 366 that is used to configure the TERM_Nresistance network of 300C. For instance, in the comparator circuit 700,the FTP signal is used to build the n-channel configurable resistance383 of the configurable second resistance configuration 365 comprising adigital portion 371 and the n-channel variable resistance 383.

Comparator circuit 300C also includes an operational amplifier 390.Inputs to the operational amplifier includes a fixed divider voltagefrom the fixed voltage divider 310, and a second divider voltage fromthe second voltage divider 360. The operational amplifier 390 generatesa bias voltage at an output. A capacitor 398 coupled to the output actsas a filter.

In particular, fixed voltage divider 310 includes resistors 305 with anode 315 that provides a fixed divider voltage. In one embodiment,resistors 305 are substantially identical, wherein in other embodiments,resistors in the fixed voltage divider 310 have different resistances.The fixed divider voltage provided at node 315 is also provided as aninput 393 to comparator or operational amplifier 390.

Another input 395 to operational amplifier 390 is coupled to a secondvoltage divider 360 that comprises a second divider voltage measured atnode 367. The second voltage divider 360 includes the reference resistorconfiguration 366, and a configurable resistor configuration 365. Moreparticularly, the second voltage divider includes a node 367 that isused as an input 395 to the comparator 390. The configurable resistorconfiguration 365 includes the digital portion 371 that is configuredthrough comparator circuit 300A, and an analog portion 383 thatcomprises a variable resistive branch coupled in parallel with thedigital portion 371. The digital portion 371 is configured similarly tothe digital portions 321 of FIGS. 3A and 3B. Specifically, the outputsof the resistor control block 322 are used to configure the digitalportion 371 of the second configurable resistor network.

Specifically, as shown in FIG. 3C, the analog portion of theconfigurable resistance is dynamically tuned through the TERM_Nresistance network 275. More particularly, the feedback loop 399 isenabled to fine tune the analog portion 383 of the configurableresistance, such that that the divider voltage provided by voltagedivider 360 matches that provided by voltage divider 310. This isaccomplished by feeding the bias voltage back to the second voltagedivider for adjusting the analog portion 383 of the configurableresistance 365. In particular, the bias voltage of the operationalamplifier 390 is fed back to the variable resistive branch of the analogportion 383.

In one embodiment, the bias voltage is applied to the variable resistivebranch of the analog portion 383 that comprises a transistor, whereinthe transistor comprises a variable resistance as determined by avariable gate voltage. That is, the output of the comparator oroperational amplifier is coupled to the transistor. In one embodiment,the transistor operates in the linear range when configuring the analogportion of the configurable resistance.

Since the analog fine tune is continuous and dynamic, the analog finetune is responsive to further fluctuations in temperature and voltagethat affect the self calibrating resistance circuits 200. For instance,in FIG. 6, the FTN2 signal provides an output of the comparator that isused to configure the analog portion 383 of the configurable resistance365.

In another embodiment, rather than establish another comparator circuitfor the TERM_N resistance network 275, voltages and current from theTERM_P resistance network 275, and/or the output generated by the TERM_Pcomparator circuit 300B of FIG. 3B, is mirrored to establish the analogportion of a configurable resistance. That is, a current mirror biasmirrors the bias from the analog FTP signal generated from the TERM_Presistance circuit 270 of FIG. 2 to form a mirrored FTN signal. Forinstance, as shown in the comparator circuit 600 of FIG. 6, a biasplaced on FTP creates a current through transistor NPFTN 650. Thiscurrent flows down through an enable switch NPOFF 653, and down throughNNFTN 657 to create a self bias voltage. As such, the current flowingthrough the two transistors 650 and 657 match. Any change to FTP will bemirrored in a corresponding change in FTN (e.g., the voltages will trackeach other through the transistors 650 and 657).

FIG. 4A is a circuit diagram of a comparator circuit 400A used forconfiguring a digital portion of a configurable resistor that isreferenced to ground or Vss, in accordance with one embodiment of theinvention. For instance, comparator circuit 400A is a stand-alonecircuit that acts independently of the comparator circuits in FIGS.3A-C. As an example, comparator circuit 400A may be used in conjunctionwith a pull down resistor (e.g., n-channel). As such, comparator circuit400A is used in a self calibrating resistance circuit for coarsely anddigitally converging a digital portion of a resistor network. In oneimplementation comparator circuit 300A is used within the comparator 255of FIG. 2. In another implementation, comparator circuit 300A is usedwithin comparator circuit 600 of FIG. 6.

Specifically, the output of the comparator circuit 400A is used toconfigure the digital portion of the configurable resistor 421. This isaccomplished by comparing divider voltages between a fixed dividervoltage and a second divider voltage. The comparator circuit 300A isimplementable within the comparator 255 of FIG. 2, in one embodiment. Inparticular, a fixed voltage divider 410 comprises a fixed dividervoltage measurable at node 415. The fixed voltage divider comprises afirst resistive component and a second resistive component. In oneimplementation, the fixed voltage divider includes resistors 405 with anode 415 that is used as an input 435 to the comparator 430. In oneembodiment, the resistors in the fixed voltage divider 310 comprises aone-to-one ratio, wherein the resistors 405 are substantially identical.In another embodiment, the resistors in the fixed voltage divider 310comprises a fixed ratio that is not one-to-one, wherein the resistorsare not identical.

Input 435 to the comparator 330 is coupled to a second voltage divider420 that comprises a second divider voltage measured at node 427. Thesecond voltage divider 320 includes a reference resistor 425, such as anexternal precision reference resistor. In addition, the second voltagedivider 420 includes a configurable resistor that includes a digitalportion 421 that is configured through comparator circuit 400A. Thedigital portion 421 comprises a plurality of selectable resistivebranches arranged in a parallel configuration, wherein adding aresistive branch reduces the resistance of the digital portion 321,whereas removing a resistive branch increases the resistance of thedigital portion 321.

The comparator 430 is configured to compare the second divider voltageand the fixed divider voltage to generate an output signal. The outputsignal is delivered to the resistor control block 422 in the digitalmode to configure the digital portion 421 of the configurableresistance. In one embodiment, resistor control block 422 performs thesame functions as resistor control block 260 of FIG. 2. In oneembodiment, the resistor control block 422 includes digital convergencecircuitry for adding or removing one of the plurality of parallelresistive branches used for configuring the digital portion 421 of theconfigurable resistor as determined by the output signal. In anotherembodiment, the resistor control block 422 includes a register forconfiguring the digital portion 421. In still another embodiment, theregister comprises a shift register for configuring the digital portion421 of the configurable resistor as determined by the output signal.

In particular, the digital convergence circuitry adds a resistive branchwhen and as long as the output signal indicates that the second dividervoltage measured at node 427 is higher than the fixed divider voltagemeasured at node 415. Also, the digital convergence circuitry removes adigital resistive branch when the output signal indicates the seconddivider voltage measured at node 427 is lower than the reference dividervoltage measured at node 415.

At a point when the output signal first indicates that the seconddivider voltage at node 427 is lower than the reference divider voltagemeasured at node 415, the resistor control block 422 removes a digitalleg back from the digital portion and then locks the digital portion ofthe configurable resistor. At that time, the voltage measured at node427 is again higher than the voltage at node 415, thereby indicatingthat the digital portion of the configurable resistance is again higherthan the reference resistance. That is, the digital convergencecircuitry digitally converges after a last resistive branch is added,which results in the output signal from the comparator 430 indicatingthat the second divider voltage at node 427 is lower than the fixeddivider voltage at node 415, and when the last resistive branch isthereafter removed so that the configurable resistance is again greaterthan the reference resistance, which results in the output signal fromthe comparator 430 indicating that the voltage at node 427 is againhigher than the voltage at node 415.

Thereafter, analog fine tuning is performed in order to dynamicallyadjust the configurable resistance. In one implementation, the digitalconvergence circuitry, registers, and/or shift registers converges tothe digital portion before the analog fine tuning is performed. In oneembodiment, for purposes of fine tuning, the analog portion is used inconjunction with the digital portion, and comprises a transistor (e.g.,metal oxide semiconductor) that has a variable resistance depending on agate to source voltage difference.

More specifically, FIG. 4B is a circuit diagram of a comparator circuit400B, or TERM_N resistance circuit, for configuring and fine tuning ananalog portion of the configurable resistor introduced in FIG. 4A,wherein the analog portion 443 is coupled in parallel with the digitalportion 421, in accordance with one embodiment of the presentdisclosure. Specifically, the output of the comparator circuit 400B isused to configure the analog portion of the configurable resistorthrough the TERM_N resistance circuit (e.g., 275) in order to configurea resistance that is referenced to a negative power supply or ground(e.g., Vss). This is accomplished by comparing divider voltages for aTERM_N resistance circuit. Comparator circuit 400B is implementablewithin comparator 255 of FIG. 2, in one embodiment.

Fixed voltage divider 410 includes resistors 405 with a node 415 thatprovides a fixed divider voltage. The fixed divider voltage provided atnode 415 is also provided as an input 453 to comparator 450.

Another input 455 to comparator 350 is coupled to a second voltagedivider 340 that comprises a second divider voltage measured at node427. The second voltage divider 440 includes the reference resistor 425coupled to V_(DD), and a configurable resistor 429. More particularly,the second voltage divider includes a node 447 that is used as an input455 to the comparator 450. The configurable resistor 429 includes thedigital portion 421 that is configured through comparator circuit 400A,and an analog portion 443 that comprises a variable resistive branchcoupled in parallel with the digital portion 421.

As such, inputs to the operational amplifier 450 includes a fixeddivider voltage from the fixed voltage divider 410, and a second dividervoltage from the second voltage divider 440. The operational amplifier450 generates a bias voltage at an output. A capacitor 458 coupled tothe output acts as a filter.

Specifically, as shown in FIG. 4B, the analog portion of theconfigurable resistance is dynamically tuned through the TERM_Nresistance network 275, and as shown in FIG. 4B. More particularly, thefeedback loop 459 is enabled to fine tune the analog portion 443 of theconfigurable resistance, such that that the divider voltage provided byvoltage divider 440 matches that provided by voltage divider 410. Inparticular, the bias voltage of the operational amplifier 450 is fedback to the variable resistive branch of the analog portion 443.

In one embodiment, within the digital course adjust there is a rangeselect function that selects the proper starting point range desiredwith respect to the external R_REF value. The initial starting rangeselect R value is always larger than R_REF. For instance, returning tothe reference array 700 shown in FIG. 7, the schematic block SW_REF0 720contains a range select circuit that is controlled by signals R1(S100)and R0(S150). These are used for range selection and are userselectable.

As examples of range selection, Table 800 in FIG. 8 illustrates a rangeselect feature of the self calibrating resistance circuit, in accordancewith one embodiment of the invention. The ranges provided in Table 800are provided for illustration purposes only, as embodiments are wellsuited to supporting various ranges for setting an initial value of therange select. In particular, a range selector is configurable to selecta range for setting an initial resistance of the digital portion of theconfigurable resistance, wherein the reference resistance and/or theinitial resistance selected falls within the range that is selected. Forinstance, Table 800 shows three ranges for selection. In the maximumrange, the reference resistance value falls within the range of 180-240ohms. As such, the initial value of the digital portion of theconfigurable resistance starts at the top value of the maximum range, or240 ohms. In the middle or nominal range, the reference resistance valuefalls within the range of 135-180 ohms. As such, the initial value ofthe digital portion of the configurable resistance starts at the topvalue of the nominal range, or 180 ohms. In the minimum range, thereference resistance value falls within the range of 100-135 ohms. Assuch, the initial value of the digital portion of the configurableresistance starts at the top value of the minimum range, or 135 ohms.

FIGS. 5A-B are flow diagrams 500A and 500B, illustrating methods forself calibrating to a reference resistor, in accordance with oneembodiment of the disclosure. Diagrams 500A-B are implementable withinthe self calibrating resistance circuit 200 of FIG. 2, in embodiments ofthe present disclosure. In another implementation, flow diagrams 500Aand 500B are configured to self calibrate a configurable resistorlocated on a die of an integrated circuit to an external referenceresistance, such as, that provided over a connection to the integratedcircuit. The configurable resistor comprises a resistor network that iscoupled to a connection, wherein the connection provides a signal from areference resistor. The resistor network comprises a digital portion andan analog portion of the configurable resistor.

FIG. 5A is a flow diagram 500A illustrating a method for converging adigital portion and an analog portion of a resistor network, inaccordance with one embodiment of the present disclosure. At 510 of FIG.5A, the digital resistor network is coarsely tuned. More particularly,the coarse tuning configures a digital portion of a configurableresistance that is used for self calibrating to a reference resistance.As previously described in relation to FIGS. 3A and 4A, coarse tuninginvolves the digital portion of the resistor network, withoutconsideration of the analog portion of the resistor network.

More particularly, FIG. 5B is a flow diagram 500B illustrating a methodfor digitally converging a digital portion of the resistor networkintroduced in FIG. 5A, in accordance with one embodiment of the presentdisclosure. Specifically, a comparator is used for comparing a fixeddivider voltage with a second divider voltage to generate an outputsignal at 530 of FIG. 5B. The fixed voltage divider comprises tworesistors coupled in series with a connection to V_(DD), and a middlenode that provides V_(DD)/2. The second voltage divider comprises thedigital portion of the resistor network coupled in series with thereference resistor and to V_(DD).

The output of the comparator is delivered to a resistor control block inthe digital mode, such that a resistive digital branch is added when theoutput signal indicates that the second divider voltage ratio is lowerthan the fixed divider voltage at 540 (of FIG. 5B). In addition, aninitial value of the digital portion of the configurable resistance isgreater than the reference resistance, and is lowered during coarsetuning.

At decision step 550 of FIG. 5B, the divider voltages are compared todetermine if the second divider voltage is higher than the fixed dividervoltage. If not, then another parallel resistive branch is added at 540.However, if the second divider voltage is higher than the fixed dividervoltage, then at 560 of FIG. 5B the last resistive digital branch isremoved when the output signal indicates that the second divider voltageis higher than the fixed divider voltage. This condition exists when toomany resistive branches have been added in parallel, and as such, aresistive digital branch is then removed. This provides a digitalportion of the configurable resistance that is still greater than thereference resistance. Once the digital resistor network has digitallyconverged and is coarsely tuned, the configuration of digital resistivebranches is locked for purposes of fine tuning the configurableresistance. That is, the digital portion that is in parallel with theanalog portion is fully configured.

In one embodiment, the digital portion of the configurable resistance isstored at 570 of FIG. 5B. For example, the configuration of the digitalresistive branches for the digital portion is stored in registers. Inone implementation, a “1” associated with a resistive branch indicatesthat that branch is included in the digital portion, whereas a “0”indicates that the branch is not included. Of course, the reverseimplementation may also be used to indicate whether a branch is includedor not in the digital portion. As such, since the digital portion isstored, readily accessed, and easily reconfigurable, only the finetuning of the resistor network is performed when exiting a low powerstate that configures both the analog and digital portions of theconfigurable resistance as open circuits. That is, the digital portionis reconfigured using the stored values, and fine tuning is performed totune the analog portion of the configurable resistance, as is describedbelow.

In particular, the configurable resistance can be further tuned by finetuning the analog portion of the resistor network that is coupled inparallel with the digital portion. Returning to FIG. 5A, at 520, theanalog resistor network and correspondingly the analog portion of theconfigurable resistance is fine tuned. As such, the total configurableresistance is also fine tuned, since the resistor network calibratingthe configurable resistance comprises the previously locked digitalresistor network coupled in parallel with the analog resistor network.The configurable resistance is related to the reference resistance of areference resistor provided by the connection. In one embodiment, theconfigurable resistance is substantially identical to the referenceresistance.

Fine tuning involves both the digital portion and the analog portion ofthe resistor network. However, the digital portion has been locked to avalue, as provided by the coarse tuning performed in 510. The fixeddivider voltage is compared with a second divider voltage to generate anoutput signal. Again, the fixed voltage divider comprises two resistorscoupled in series with a connection to V_(DD), and a middle node thatprovides V_(DD)/2. The second voltage divider now comprises theresistive network coupled in series with the reference resistor and toV_(DD). The resistive network comprises the digital portion and theanalog portion, wherein the analog portion is fine tuned to allow theconfigurable resistance to match the reference resistance.

Fine tuning the analog portion of the configurable resistance is aidedby the feedback loop coupling the output of the comparator to the secondvoltage divider providing an input to the comparator. In particular, abias voltage is generated through an operational amplifier that isconfigured with inputs coupled to said fixed voltage divider and saidsecond voltage divider. The bias voltage is then fed back to the secondvoltage divider for adjusting the analog portion of the configurableresistance. In one embodiment, the analog portion of the configurableresistance comprises an adjustable resistive branch that is coupled inparallel with the digital portion that is adjustable through atransistor configuration. For instance, the adjustable resistive branchcomprises a transistor with a variable resistance that is controlled byvarying the gate to source voltage over the transistor. As such, thefeedback loop continually varies the gate voltage on the transistor inorder to provide the proper proportions between the reference resistorand the configurable resistor. In one embodiment, the gate voltage isvaried on the transistor in order to match the configurable resistor tothe reference resistor, wherein the configurable resistance issubstantially the same as the reference resistance.

Moreover, the analog portion is fine tuned in a dynamic fashion. Assuch, the analog portion is continuously updated in response tofluctuations in voltage and temperature affecting the self calibratingresistance circuit performing the method outlined in FIG. 5A. As such,the configurable resistance is dynamically adjusted by dynamicallyadjusting the analog portion of the resistor network.

In another embodiment, the digital portion and the analog portion areconfigured as open circuits when in a low power state. The configurationof the digital portion has been stored before entering the low powerstate. As such, when exiting the low power state, the storedconfiguration of the digital portion is retrieved to re-enable thedigital portion of the configurable resistance. Thereafter, only finetuning need be performed to configure the configurable resistance whenexiting the low power state.

FIG. 6 is a circuit diagram of a comparator circuit 500 used in a selfcalibrating resistance circuit, in accordance with one embodiment of thedisclosure. Comparator circuit 600 implements the functions ofcomparator 255 of FIG. 2, as well as comparators found in of FIGS. 3A-Cand 4A-B to coarsely tune a digital portion of a configurable resistorand to finely tune an analog portion of the configurable resistor.

As shown, comparator circuit 600 compares a reference voltage that isgenerated by a resistor divider network to the voltage between thereference array establishing the configurable resistor and the referenceresistor. Circuit 600 operates in a coarse mode used for tuning thedigital portion of the configurable resistor, and an analog fine tunemode used for fine tuning the analog portion of the configurableresistor.

In the coarse mode, the operational amplifier 610 generates a value ofone or zero for the COMP signal depending on the voltage comparison, aspreviously described. Selection of one or more parallel, digitalresistive branches is selected during the coarse mode. Once the digitalportion of the configurable resistance is determined in coarse mode, thedigital portion is fully configured and locked for purposes of finetuning the configurable resistance.

During the analog fine tune mode, the operational amplifier 610 may beused to generate analog voltages FTP and FTN, or FTP and FTN2. Forinstance, operational amplifier 610 is used to generate FTP in a TERM_Presistive circuit (e.g., circuit 270. In particular, as shown in FIG. 6,the TERM_P resistance network including operational amplifier 610 andp-channel 650 is used for terminating a line to a power supply, such asV_(DD). The TERM_P resistance provides for an analog fine tuning of theconfigurable resistance for lines referenced to V_(DD). The resultingresistance provided by the corresponding p-channel provides for ananalog portion of the configurable resistance that is dynamically tuned,and configured in parallel with the digital portion of the configurableresistance.

FIG. 9A is a more detailed illustration of a TERM_P resistance circuit900A. In particular, p-channel transistor 950 provides for the analogportion of the configurable resistance that is dynamically tuned.P-channel 950 is configured in parallel with the digital portion of theconfigurable resistance. P-channel 950 is controlled by the FT or FTPsignal as delivered through intervening pass gates. Resistor 955 coupledto p-channel 950 helps ensure that the p-channel 950 operates in thelinear region, in one embodiment. In addition, the output of TERM_Presistance circuit 900A is coupled to a pad (not shown), and the IN lineis coupled to a positive voltage, such as, V_(DD). A feedback 960 isprovided that is used to adjust the gate voltage of the p-channel 950 inorder to tune the configurable resistance proportionally to thereference resistor.

FIG. 9B is a circuit diagram of an TERM_N resistance network forterminating a line to ground or Vss, in accordance with one embodimentof the disclosure. FIG. 9B is a more detailed illustration of a TERM-Nresistance circuit 900B. In particular, n-channel transistor 970provides for the analog portion of the configurable resistance that isdynamically tuned. N-channel 970 is configured in parallel with thedigital portion of the configurable resistance. N-channel 970 iscontrolled by the FT or FTN signal as delivered through intervening passgates. Resistor 975 coupled to n-channel 970 helps ensure that thep-channel 970 operates in the linear region, in one embodiment. Inparticular, the output is coupled to Vss or ground. In addition, theinput IN is connected to the pad (not shown), through which there iscoupling between the TERM_P resistance circuit 900A and TERM_Nresistance circuit 900B. A feedback is provided that is used to adjustthe gate voltage of the n-channel 970 in order to tune the configurableresistance proportionally to the reference resistor.

FIG. 10 is a circuit diagram of a resistor control block 1000 used forconfiguring the digital portions of the configurable resistor in a selfcalibrating resistance circuit, in accordance with one embodiment of thedisclosure. For instance, resistor control block 1000 performs the samefunctions as control blocks 322 and 422 of FIGS. 3B and 4B,respectively, and are used to digitally converge the digital portions ofrespective configurable resistances.

FIG. 11 is a signal diagram illustrating waveforms of simulation resultswhen implementing a self calibrating resistance network, in accordancewith one embodiment of the present disclosure. For instance, signalwaveform 1110 shows the digital and analog convergence of a configurableresistance. As shown, the reference resistance is approximately 240ohms. The initial resistance of the configurable resistance is greaterthan 240 at point 1115 and digitally converges within 2 or 3 clocks. Atpoint 1120, the transition between the digital mode and the analog modeis shown. For example, the digital resistance has gone too low and isincreased by removing a parallel resistive branch in the digitalportion. Also, the quick convergence of the analog fine tuning is shown,such that the configurable resistance is at approximately 240 ohms. Inaddition, at 1125, there is a step down of resistance values by one-halfto 120 ohms, such that other resistances can be provided throughout theI/O circuit. Also, at 1130, there is a further step down of resistancevalues to one-third the original resistance value, to 80 ohms, of theconfigurable resistance. Other waveforms 1140 and 1150 illustrate thecalibration to reference resistances of approximately 200 and 160 ohms,respectively.

FIG. 12 is a circuit diagram illustrating a step down feature of theself calibrating resistance circuit, in accordance with one embodimentof the disclosure. As shown, three configurable resistance circuits areshown coupled in parallel. Selection of one or more of the configurableresistance circuits 1250 will determine the overall resistance value.For instance, using only one configurable resistance circuit 1250provides for a full resistance value (e.g., 240 ohms). However, couplingtwo configurable resistance circuits 1250 together results in a stepdown of one-half of the total resistance value (e.g., 120 ohms). Also,coupling three configurable resistance circuits 1250 together results ina step down of one-third of the total resistance value (e.g., 80 ohms).

Thus, according to embodiments of the present disclosure, circuits andmethods for self calibrating to a reference resistance are described.Embodiments of the present invention provide for circuits and methodsthat self calibrate to a reference resistance provided by a connectionto a configurable resistor network that comprises a digital portion forcoarse tuning the configurable resistor network and an analog portionfor dynamic fine tuning of the configurable resistor network.

While the foregoing disclosure sets forth various embodiments usingspecific block diagrams, flow charts, and examples, each block diagramcomponent, flow chart step, operation, and/or component described and/orillustrated herein may be implemented, individually and/or collectively.In addition, any disclosure of components contained within othercomponents should be considered as examples because many otherarchitectures can be implemented to achieve the same functionality.

The process parameters and sequence of steps described and/orillustrated herein are given by way of example only and can be varied asdesired. For example, while the steps illustrated and/or describedherein may be shown or discussed in a particular order, these steps donot necessarily need to be performed in the order illustrated ordiscussed. The various example methods described and/or illustratedherein may also omit one or more of the steps described or illustratedherein or include additional steps in addition to those disclosed.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as may be suited to theparticular use contemplated.

Embodiments according to the invention are thus described. While thepresent disclosure has been described in particular embodiments, itshould be appreciated that the invention should not be construed aslimited by such embodiments, but rather construed according to the belowclaims.

The invention claimed is:
 1. A self calibrating resistance circuit,comprising: a connection providing a signal from a reference resistor; aresistor network coupled to said connection, wherein said resistornetwork provides a configurable resistance across said connection,wherein said resistor network comprises a digital resistor network andan analog resistor network; and control circuitry for configuring saidconfigurable resistance based on a reference resistance of saidreference resistor by coarsely tuning said resistor network through saiddigital resistor network and fine tuning said resistor network throughsaid analog resistor network.
 2. The circuit of claim 1, wherein saidresistor network and said control circuitry is included within anintegrated circuit that is coupled to said connection.
 3. The circuit ofclaim 2, wherein said configurable resistance comprises an internaltermination resistance.
 4. The circuit of claim 2, wherein saidreference resistor comprises an external reference resistor.
 5. Thecircuit of claim 1, wherein said configurable resistance issubstantially the same as said reference resistance.
 6. The circuit ofclaim 1, wherein said configurable resistance comprises: a digitalportion of said configurable resistance configured by said digitalresistor network; and an analog portion of said configurable resistanceconfigured by said analog resistor network, wherein said digital portionand said analog portion are coupled in parallel.
 7. The circuit of claim6, wherein said analog resistor network is configured to dynamicallyadjust said configurable resistance by dynamically tuning said analogportion of said configurable resistance.
 8. The circuit of claim 6,wherein said control circuitry comprises: wherein said digital resistornetwork comprises a plurality of selectable, parallel resistivebranches; and a fixed voltage divider generating a fixed dividervoltage; and a second voltage divider comprising said digital portion ofsaid configurable resistance, said reference resistance, wherein saidsecond voltage divider generates a second divider voltage; saidcomparator for comparing said second divider voltage and said fixeddivider voltage to generate an output signal; and a resistor controlblock for configuring said digital portion of said configurableresistance based on said output signal.
 9. The circuit of claim 6,wherein said control circuitry comprises: said analog resistor networkcomprising an adjustable resistive branch in parallel with said digitalportion of said configurable resistance; an operational amplifier havinginputs of a fixed voltage divider generating a fixed divider voltage anda second voltage divider comprising said analog resistor network andsaid reference resistor, wherein said second voltage divider generates asecond divider voltage, wherein said operational amplifier generates abias voltage at an output; and a feedback loop for said operationalamplifier feeding said bias voltage back to said second voltage dividerfor adjusting said analog portion of said configurable resistance. 10.The circuit of claim 1, further comprising: a power controller forconfiguring said digital portion and said analog portion of saidresistor network as open circuits when in a low power state.
 11. Thecircuit of claim 1, further comprising a second self calibratingresistance circuit comprising: a second resistor network providing asecond configurable resistance, said second resistance networkcomprising a second digital resistor network and a second analogresistor network, wherein said second digital resistor network isconfigured similarly as said digital resistor network; and a secondcontrol circuitry for configuring said second configurable resistancebased on a second reference resistance by fine tuning said secondresistance network through said second analog resistor network, whereinsaid second reference resistance is configured similarly as saidresistor network.
 12. A self calibrating resistance circuit, comprising:a connection providing a signal from a reference resistor; a resistornetwork coupled to said connection, wherein said resistor networkprovides a configurable resistance across said connection and comprises:a digital resistor network for configuring a digital portion of saidconfigurable resistance across said connection; and an analog resistornetwork for configuring an analog portion of said configurableresistance across said connection, wherein said digital portion and saidanalog portion are coupled in parallel; control circuitry forconfiguring said configurable resistance based on a reference resistanceof said reference resistor by coarsely tuning said resistor networkthrough said digital resistor network and for dynamically fine tuningsaid resistor network through said analog resistor network; and a rangeselector for configuring a range for setting an initial resistance ofsaid digital portion of said configurable resistance.
 13. The circuit ofclaim 12, wherein said control circuitry converges to said digitalportion before performing said dynamically fine tuning of said analogresistor network.
 14. The circuit of claim 12, wherein said initialresistance of said digital portion is greater than said referenceresistance.
 15. The circuit of claim 12, further comprising: anintegrated circuit comprising said resistor network and said controlcircuitry, wherein said integrated circuit is coupled to saidconnection.
 16. The circuit of claim 12, wherein said configurableresistance is substantially the same as said reference resistance. 17.The circuit of claim 12, wherein said analog resistor network isconfigured to dynamically adjust said configurable resistance bydynamically tuning said analog portion of said configurable resistance.18. The circuit of claim 12, wherein said control circuitry forconfiguring said digital resistor network comprises: wherein saiddigital resistor network comprises a plurality of selectable, parallelresistive branches; and a fixed voltage divider generating a fixeddivider voltage; and a second voltage divider comprising said digitalportion of said configurable resistor, said reference resistor, whereinsaid second voltage divider generates a second divider voltage; saidcomparator for comparing said second divider voltage and said fixeddivider voltage to generate an output signal; and a resistor controlblock for configuring said digital portion of said configurableresistance based on said output signal.
 19. The circuit of claim 18,wherein said resistor control block comprises: a register for adding orremoving one of said plurality of parallel resistive branches asdetermined by said output signal, wherein shift register adds aresistive branch when said output signal indicates said second dividervoltage is lower than said fixed divider voltage and removes a resistivebranch when said output signal indicates said second divider voltage ishigher than said reference divider voltage.
 20. The circuit of claim 12,wherein said control circuitry for configuring said analog resistornetwork comprises: said analog resistor network comprising an adjustableresistive branch in parallel with said digital portion of saidconfigurable resistance; an operational amplifier having inputs of afixed voltage divider generating a fixed divider voltage and a secondvoltage divider comprising said analog resistor network and saidreference resistor, wherein said operational amplifier generates a biasvoltage at an output; and a feedback loop for said operational amplifierfeeding said bias voltage back to said second voltage divider foradjusting said analog portion of said configurable resistance.
 21. Amethod for self calibrating a resistance, comprising: coarsely tuning adigital resistor network to configure a digital portion of aconfigurable resistance; and fine tuning an analog resistor network toconfigure an analog portion of said configurable resistance, wherein aresistor network comprises said digital resistor network and said analogresistor network coupled in parallel to configure said configurableresistance, wherein said configurable resistance is related to areference resistance provided by a connection.
 22. The method of claim21, wherein said coarsely tuning comprises: comparing a fixed dividervoltage configured by a fixed voltage divider and a second dividervoltage configured by a second voltage divider comprising said digitalportion of said configurable resistance and said reference resistance;adding one of a plurality of parallel resistive branches when saidsecond divider voltage is lower than said fixed divider voltage, whereinsaid digital resistor network comprises said plurality of parallelresistive branches; and removing one of said plurality of parallelresistive branches when said second divider voltage is higher than saidfixed divider voltage.
 23. The method of claim 22, wherein said coarselytuning further comprises: determining when said second divider voltageis higher than said fixed divider voltage after a last parallelresistive branch is added; digitally converging said digital portion byremoving said last parallel resistive branch; and storing aconfiguration of said digital portion.
 24. The method of claim 21,further comprising: configuring an initial value of said digital portionof said configurable resistance to be greater than a referenceresistance of said reference resistor.
 25. The method of claim 21,wherein said analog fine tuning comprises: dynamically adjusting saidanalog portion of said configurable resistance in order to dynamicallyadjust said configurable resistance.
 26. The method of claim 25, furthercomprising: generating a bias voltage through an operational amplifierconfigured with inputs coupled to said fixed voltage divider and saidsecond voltage divider; and feeding said bias voltage back to saidsecond voltage divider for adjusting said analog portion of saidconfigurable resistance, wherein said analog portion comprises anadjustable resistive branch coupled in parallel with said digitalportion of said configurable resistance.
 27. The method of claim 21,further comprising: configuring said configurable resistance as beingsubstantially the same as said reference resistance.
 28. The method ofclaim 21, further comprising: configuring said digital portion and saidanalog portion of said configurable resistance as an open circuit whenin a low power state.
 29. The method of claim 28, further comprising:retrieving a stored configuration of said digital portion to re-enablesaid digital portion of said configurable resistance; and performingsaid fine tuning of said resistor network to configure said configurableresistance when exiting said low power state.
 30. The method of claim21, further comprising: providing a secondary resistance that is a ratioof said configurable resistance.